Basal cAMP Research Articles

Changes in Membrane Cholesterol Differentially Influence Preferential and Non-preferential Signaling of the M1 and M3 Muscarinic Acetylcholine Receptors.

We have found earlier that changes in membrane cholesterol content have distinct impact on signaling via the M1, M2, or M3 receptors expressed in CHO cells (CHO-M1 through CHO-M3). Now we investigated whether gradual changes in membrane cholesterol exerts differential effects on coupling of the M1 and M3 muscarinic receptors to preferential signaling pathways through Gq/11 and non-preferential Gs G-proteins signaling. Changes in membrane cholesterol resulted in only marginal alterations of antagonist and agonist affinity of the M1 and M3 receptors, and did not influence precoupling of either subtype. Changes in membrane cholesterol did not influence parameters of carbachol-stimulated GTP-γ35S binding in CHO-M1 membranes while reduction as well as augmentation of membrane cholesterol lowered the efficacy but increased the potency of carbachol in CHO-M3 membranes. Gradual increase or decrease in membrane cholesterol concentration dependently attenuated agonist-induced inositolphosphates release while only cholesterol depletion increased basal values in both cell lines. Similarly, membrane cholesterol manipulation modified basal and agonist-stimulated cAMP synthesis via Gs in the same way in both cell lines. These results demonstrate that changes in membrane cholesterol concentration differentially impact preferential and non-preferential M1 and M3 receptor signaling. They point to the activated G-protein/effector protein interaction as the main site of action in alterations of M1 receptor-mediated stimulation of second messenger pathways. On the other hand, modifications in agonist-stimulated GTP-γ35S binding in CHO-M3 membranes indicate that in this case changes in ligand-activated receptor/G-protein interaction may also play a role.

Role of membrane microdomains in compartmentation of cAMP signaling.

Spatially restricting cAMP production to discrete subcellular locations permits selective regulation of specific functional responses. But exactly where and how cAMP signaling is confined is not fully understood. Different receptors and adenylyl cyclase isoforms responsible for cAMP production are not uniformly distributed between lipid raft and non-lipid raft domains of the plasma membrane. We sought to determine the role that these membrane domains play in organizing cAMP responses in HEK293 cells. The freely diffusible FRET-based biosensor Epac2-camps was used to measure global cAMP responses, while versions of the probe targeted to lipid raft (Epac2-MyrPalm) and non-raft (Epac2-CAAX) domains were used to monitor local cAMP production near the plasma membrane. Disruption of lipid rafts by cholesterol depletion selectively altered cAMP responses produced by raft-associated receptors. The results indicate that receptors associated with lipid raft as well as non-lipid raft domains can contribute to global cAMP responses. In addition, basal cAMP activity was found to be significantly higher in non-raft domains. This was supported by the fact that pharmacologic inhibition of adenylyl cyclase activity reduced basal cAMP activity detected by Epac2-CAAX but not Epac2-MyrPalm or Epac2-camps. Responses detected by Epac2-CAAX were also more sensitive to direct stimulation of adenylyl cyclase activity, but less sensitive to inhibition of phosphodiesterase activity. Quantitative modeling was used to demonstrate that differences in adenylyl cyclase and phosphodiesterase activities are necessary but not sufficient to explain compartmentation of cAMP associated with different microdomains of the plasma membrane.

Microarray analysis of the effects of elevated cAMP in differentiating osteoblasts (1013.6)

McCune‐Albright Syndrome (MAS) is an acquired genetic disorder caused by a mutation in Gs alpha at Arg201 that inhibits GTP hydrolysis, constitutively activating the protein and resulting in elevated levels of basal cAMP in the affected cells. A major pathological feature of MAS is polyostotic fibrous dysplasia of bone (FD), a finding for which very little effective therapy exists. In an effort to identify novel potential drug targets to treat FD, we utilized a human fetal osteoblast cell line, hFOB1.19 (ATCC) that proliferates at 33C and differentiates into mature osteoblasts at 39C. We treated cells with the fungal metabolite forskolin at its EC50 concentration for elevating cAMP in these cells, and then either maintained the cells at 33C or shifted them to the differentiation conditions for 24 hours. Total RNA was collected and analyzed for levels of gene expression using an Illumina HT12 bead chip. Pairwise analysis of changes in gene expression identified 370 genes that were aberrantly expressed during differentiation in the presence of elevated cAMP. Twenty‐eight genes with known functions in osteoblast biology and/or differentiation were further analyzed by RT‐PCR to confirm the expression changes seen in the microarray assay.Grant Funding Source: Supported by NIH grant 1R15ED020190‐01

Characterizing intragenic suppressors of constitutively active alleles of Gs alpha (802.6)

Constitutive activity of Gs alpha is associated with several human diseases, including McCune‐Albright Syndrome (MAS). In this genetic disorder, Arg201 is mutated to His, Cys, or Ser, resulting in inhibition of GTP hydrolysis and subsequent overactivation of adenylyl cyclase and elevated basal cAMP levels. Previous studies identified three intragenic suppressor sites that blocked the constitutive activity caused by R201H: F142, R231, and L266. These mutations did not by themselves alter the activity of Gs alpha. The intragenic suppressors were examined for their ability to block the other MAS alleles. F142S and R231C suppressed the constitutive activity of both of MAS alleles. L266N suppressed R201S, and is currently being tested for the ability to block the constitutive activity of R201C. Thus, the intragenic suppressors may be useful drug targeting sites for MAS mutations generally, not just for R201H‐caused cases of the disease.Grant Funding Source: Supported by NIH grant 1R15ED020190‐01

PDE2-mediated cAMP hydrolysis accelerates cardiac fibroblast to myofibroblast conversion and is antagonized by exogenous activation of cGMP signaling pathways

Recent studies suggest that the signal molecules cAMP and cGMP have antifibrotic effects by negatively regulating pathways associated with fibroblast to myofibroblast (MyoCF) conversion. The phosphodiesterase 2 (PDE2) has the unique property to be stimulated by cGMP, which leads to a remarkable increase in cAMP hydrolysis and thus mediates a negative cross-talk between both pathways. PDE2 has been recently investigated in cardiomyocytes; here we specifically addressed its role in fibroblast conversion and cardiac fibrosis. PDE2 is abundantly expressed in both neonatal rat cardiac fibroblasts (CFs) and cardiomyocytes. The overexpression of PDE2 in CFs strongly reduced basal and isoprenaline-induced cAMP synthesis, and this decrease was sufficient to induce MyoCF conversion even in the absence of exogenous profibrotic stimuli. Functional stress-strain experiments with fibroblast-derived engineered connective tissue (ECT) demonstrated higher stiffness in ECTs overexpressing PDE2. In regard to cGMP, neither basal nor atrial natriuretic peptide-induced cGMP levels were affected by PDE2, whereas the response to nitric oxide donor sodium nitroprusside was slightly but significantly reduced. Interestingly, despite persistently depressed cAMP levels, both cGMP-elevating stimuli were able to completely prevent the PDE2-induced MyoCF phenotype, arguing for a double-tracked mechanism. In conclusion, PDE2 accelerates CF to MyoCF conversion, which leads to greater stiffness in ECTs. Atrial natriuretic peptide- and sodium nitroprusside-mediated cGMP synthesis completely reverses PDE2-induced fibroblast conversion. Thus PDE2 may augment cardiac remodeling, but this effect can also be overcome by enhanced cGMP. The redundant role of cAMP and cGMP as antifibrotic meditators may be viewed as a protective mechanism in heart failure.

Compartmentation of Camp Signaling in Complex and Simple Cells

Differences in compartmentation of cAMP signaling were compared in adult rat cardiac ventricular myocytes and HEK293 cells. The freely diffusible Epac2-camps FRET-based biosensor was used to monitor cAMP responses in the bulk cytoplasmic compartment of these cells, while Epac2-MyrPalm and Epac2-CAAX versions of the probe were used to measure subcellular cAMP responses associated with lipid raft and non-lipid raft domains of the plasma membrane, respectively. Stimulating raft associated beta-adrenergic receptors (βAR) or non-raft associated E-type prostaglandin receptors (EPR) elicited markedly different cAMP responses in the two cell types. In HEK293 cells, maximal βAR or EPR stimulation produced saturating cAMP responses in all three domains. In cardiac myocytes, maximal βAR stimulation produced non-saturating responses in all three domains. However, EPR stimulation produced responses that were smaller, transient, and more consistently observed in non-lipid raft and bulk cytoplasmic domains. There were also significant differences in the pattern of basal cAMP activity associated with the different microdomains of the two cell types. Direct inhibition of adenylyl cyclase (AC) activity with MDL12330A (MDL) only produced a decrease in basal cAMP activity in non-lipid raft domains of HEK293 cells. However, MDL inhibited basal cAMP activity in non-lipid raft domains as well in the bulk cytoplasmic compartment of cardiac myocytes. In cardiac myocytes, responses detected in all three locations were significantly more sensitive to inhibition of phosphodiesterase (PDE) activity. However, in HEK293 cells, responses detected in non-lipid raft domains were more sensitive to direct stimulation of AC activity with forskolin. Computational modeling suggests that AC activity may play a more important role in explaining the compartmentalized responses observed in HEK293 cells, while PDE activity may be more important in cardiac myocytes.

Precocious Puberty and Leydig Cell Hyperplasia in Male Mice With a Gain of Function Mutation in the LH Receptor Gene

The LH receptor (LHR) is critical for steroidogenesis and gametogenesis. Its essential role is underscored by the developmental and reproductive abnormalities that occur due to genetic mutations identified in the human LHR. In males, activating mutations are associated with precocious puberty and Leydig cell hyperplasia. To generate a mouse model for the human disease, we have introduced an aspartic acid to glycine mutation in amino acid residue 582 (D582G) of the mouse LHR gene corresponding to the most common D578G mutation found in boys with familial male-limited precocious puberty (FMPP). In transfected cells, mouse D582G mLHR exhibited constitutive activity with a 23-fold increase in basal cAMP levels compared with the wild-type receptor. A temporal study of male mice from 7 days to 24 weeks indicated that the knock-in mice with the mutated receptor (KiLHR(D582G)) exhibited precocious puberty with elevated testosterone levels as early as 7 days of age and through adulthood. Leydig cell-specific genes encoding LHR and several steroidogenic enzymes were up-regulated in KiLHR(D582G) testis. Leydig cell hyperplasia was detected at all ages, whereas Sertoli and germ cell development appeared normal. A novel finding from our studies, not previously reported in the FMPP cases, is that extensive hyperplasia is commonly found around the periphery of the testis. We further demonstrate that the hyperplasia is due to premature proliferation and precocious differentiation of adult Leydig cells in the KiLHR(D582G) testis. The KiLHR(D582G) mice provide a mouse model for FMPP, and we suggest that it is a useful model for studying pathologies associated with altered LHR signaling.

The possible roles of hyperpolarization-activated cyclic nucleotide channels in regulating pacemaker activity in colonic interstitial cells of Cajal

BackgroundHyperpolarization-activated cyclic nucleotide (HCN) channels are pacemaker channels that regulate heart rate and neuronal rhythm in spontaneously active cardiac and neuronal cells. Interstitial cells of Cajal (ICCs) are also spontaneously active pacemaker cells in the gastrointestinal tract. Here, we investigated the existence of HCN channel and its role on pacemaker activity in colonic ICCs.MethodsWe performed whole-cell patch clamp, RT-PCR, and Ca2+-imaging in cultured ICCs from mouse mid colon.ResultsSQ-22536 and dideoxyadenosine (adenylate cyclase inhibitors) decreased the frequency of pacemaker potentials, whereas both rolipram (cAMP-specific phosphodiesterase inhibitor) and cell-permeable 8-bromo-cAMP increased the frequency of pacemaker potentials. CsCl, ZD7288, zatebradine, clonidine (HCN channel blockers), and genistein (a tyrosine kinase inhibitor) suppressed the pacemaker activity. RT-PCR revealed expression of HCN1 and HCN3 channels in c-kit and Ano1 positive colonic ICCs. In recordings of spontaneous intracellular Ca2+ [Ca2+]i oscillations, rolipram and 8-bromo-cAMP increased [Ca2+]i oscillations, whereas SQ-22536, CsCl, ZD7288, and genistein decreased [Ca2+]i oscillations.ConclusionsHCN channels in colonic ICCs are tonically activated by basal cAMP production and participate in regulation of pacemaking activity.

Daily rhythms in locomotor circuits in Drosophila involve PDF

The neuropeptide pigment-dispersing factor (PDF) has been studied extensively in Drosophila, and its role in circadian time-keeping has been firmly established. The role of PDF outside of the clock circuit, however, is poorly understood. A recent study suggested that PDF may act on the ellipsoid body (EB) to link the clock and sleep/activity circuits. We performed whole brain optical imaging with the fluorescence resonance energy transfer (FRET)-based cAMP sensor Epac1-camps expressed under control of the pdfR promoter to address how the clock and sleep deprivation affect the physiology of these cells. Basal cAMP levels in EB were regulated both by PDF and synaptic inputs that are controlled by the circadian clock. Acute application of PDF to the brain caused a significant, and PDF-receptor-dependent, increase in cAMP in EB cells. Application of TTX to block circuit-mediated effects of PDF increased the morning response but not the response at night, implying the existence of a temporally regulated, PDF-stimulated input that blocks cAMP generation. ACh produced both direct (TTX-insensitive) and indirect (TTX-sensitive) increases in cAMP during the day but was totally TTX-insensitive at night, indicating that ACh-stimulated inputs to the EB are suppressed at night. Sleep deprivation did not affect the cAMP responses of these cells to either PDF or ACh. These results suggest a novel role for PDF as a modulator of activity outside of the clock circuit. By elucidating the mechanisms by which the neuropeptide PDF act on its target cells, our work contributes to our understating of how the central clock coordinates activity and sleep.

Calcium supplementation increases circulating cholesterol by reducing its catabolism via GPER and TRPC1-dependent pathway in estrogen deficient women

Limited studies have addressed the effects of calcium supplementation (CaS) on serum total cholesterol (TC) in postmenopausal women and the results are inconclusive. Moreover, the potential mechanisms through which CaS regulates cholesterol metabolism in the absence of estrogen are still sealed for the limitation of human being study. Cross-sectional survey, animal and in vitro experiments were conducted to investigate the effect of CaS on endogenous cholesterol metabolism in estrogen deficiency and identify its potential mechanisms. Ovariectomized rats were used to mimic estrogen deficiency. In vitro, HepG2 cell line was exposed to estradiol and/or calcium treatment. We demonstrated that CaS significantly increased serum TC and the risk of hypercholesterolemia and myocardial infarction in postmenopausal women. Increased serum TC in estrogen deficiency was caused mainly by decreased cholesterol catabolism rather than increased synthesis. This was mediated by reduced 7α-hydroxylase resulting from increased liver intracellular Ca(2+) concentrations, reduced intracellular basal cAMP and subsequent up-regulation of SREBP-1c and SHP expression. Estrogen had a protective role in preventing CaS-induced TC increase by activating the G-protein coupled estrogen receptor, which mediated the estrogen effect through the transient receptor potential canonical 1 cation channel. CaS increases endogenous serum TC via decreasing hepatic cholesterol catabolism in estrogen deficiency. G-protein coupled estrogen receptor is shown to be a key target in mediating CaS-induced TC increase. CaS should be monitored for the prevention of serum TC increase during menopause.

The W520X mutation in the TSHR gene brings on subclinical hypothyroidism through an haploinsufficiency mechanism.

TSHR is a G-protein-coupled seven transmembrane domain receptor that activates the two major signal transduction pathways: the Gαs/adenylate cyclase and the Gαq/11/phospholipase C pathways. Inactivating mutations in the TSHR gene have been demonstrated to be responsible for subclinical hypothyroidism, a disorder characterized by elevated serum TSH concentrations despite normal thyroid hormones levels. We identified in a child a nonsense mutation (W520X) in the third transmembrane domain of the TSHR that causes the lack of the C-terminus portion of the receptor. The functional significance of this variation was assessed in vitro. The W520X mutation was introduced into the pSVL vector containing the wild-type sequence of TSHR gene. Wild-type and mutated vectors were expressed in Chinese Hamster Ovary (CHO) cells, and cAMP, inositol phosphate (IP), immunofluorescence and FACS analyses were performed. Transfection with pSVL-TSHR vector induced basal cAMP and IP production in the absence of TSH stimulation, indicating a constitutive activity for the TSHR. An impairment of receptor function was demonstrated by the observation that cells expressing the mutant TSHR exhibited a lower second messenger production with respect to the wild-type, despite a normal expression of the receptor at the cell surface. The mechanism through which the W520X mutation exerts its effect is more likely haploinsufficiency rather than a dominant-negative effect. This could explain the phenotype of our patient, who has a hormonal pattern in the range of a mild subclinical hypothyroidism, without an overt disease phenotype.

Omeprazole inhibits biliary proliferation during extrahepatic cholestasis

Omeprazole (OME) has been shown to reduce liver regeneration and enhance gastrin serum levels. Since gastrin inhibits biliary growth, we tested the hypothesis that omeprazole inhibits cholangiocyte proliferation in BDL rats by increasing gastrin serum levels.MethodsIn vivo, male Fischer 344 rats (immediately BDL) were treated with vehicle or OME (40 mg/Kg BW daily) for 1 wk. We measured changes in: liver histology, and intrahepatic bile duct mass in liver sections; H3 histone and PCNA and secretin receptor (SR) gene expression; basal and secretin‐stimulated cAMP levels in cholangiocytes bile and bicarbonate secretion in bile fistula rats. Changes in SR and secretin‐stimulated secretion are key functional tools for evaluating the degree of biliary hyperplasia.ResultsChronic in vivo administration of OME to BDL rats increased gastrin serum levels and reduced biliary proliferation and hyperplasia and decreased SR expression and significantly inhibited the stimulatory effects of secretin on cAMP levels and ductal secretion compared to vehicle treated controls.ConclusionThe usage of OME may be important for the management of chronic cholestatic liver diseases.

A Small Molecule Antagonist Inhibits Thyrotropin Receptor Antibody-Induced Orbital Fibroblast Functions Involved in the Pathogenesis of Graves Ophthalmopathy

Graves ophthalmopathy (GO) is an autoimmune disorder characterized by increased adipogenesis and hyaluronan (HA) production by orbital fibroblasts. Circulating autoantibodies (thyroid-stimulating antibodies [TSAbs]) directed at the thyrotropin receptor (TSHR) on these cells stimulate or augment these cellular processes. A recently developed drug-like small molecule inverse agonist of TSHR, NCGC00229600, termed 1, binds to TSHR and blocks basal and stimulated signal transduction. The purpose of this article was to determine whether 1 might inhibit HA production and relevant signaling pathways in orbital fibroblasts cultured in the presence of monoclonal TSAbs or bovine TSH (bTSH). Primary cultures of undifferentiated GO orbital fibroblasts (n = 13) were untreated or treated with a TSAb (M22 or MS-1) or bTSH in serum-free medium, with or without 1 or a TSHR neutral antagonist, NCGC00242595, termed 2, which does not inhibit basal signaling but does inhibit stimulated signaling. cAMP production, Akt phosphorylation (Ser473pAkt in media and immunoblotting for pAkt/total Akt), and HA production were analyzed. Compound 1 inhibited basal cAMP, pAkt, and HA production and that stimulated by M22 in undifferentiated orbital fibroblasts. Inhibition of HA production was dose-dependent, with a half-maximal inhibitory dose of 830 nM. This compound also inhibited MS-1- and bTSH-stimulated cAMP, pAkt, and HA production. Compound 2 did not inhibit basal HA production but did inhibit M22-stimulated HA production. Because cAMP, pAkt, and HA production are fibroblast functions that are activated via TSHR signaling and are important in the pathogenesis of GO, small molecule TSHR antagonists may prove to be effective in the treatment or prevention of the disease in the future.

Selective inhibition of cell death in malignant vs normal B-cell precursors: implications for cAMP in development and treatment of BCP-ALL

AbstractB-cell precursor acute lymphoblastic leukemia (BCP-ALL) is the most commonly occurring pediatric cancer. Despite its relatively good prognosis, there is a steady search for strategies to improve treatment effects and prevent the undesired side effects on normal cells. In the present paper, we demonstrate a differential effect of cyclic adenosine monophosphate (cAMP) signaling between normal BCPs and BCP-ALL blasts, pointing to a potential therapeutic window allowing for manipulation of cAMP signaling in the treatment of BCP-ALL. By studying primary cells collected from pediatric BCP-ALL patients and healthy controls, we found that cAMP profoundly decreased basal and DNA damage-induced p53 levels and cell death in malignant cells, whereas normal BCP counterparts displayed slightly augmented cell death when exposed to cAMP-increasing agents. We did not find evidence for a selection process involving generation of increased basal cAMP levels in BCP-ALL cells, but we demonstrate that paracrine signaling involving prostaglandin E2-induced cAMP generation has the potential to suppress p53 activation and cell death induction. The selective inhibitory effect of cAMP signaling on DNA damage-induced cell death in BCP-ALL cells appears to be an acquired trait associated with malignant transformation, potentially allowing the use of inhibitors of this pathway for directed killing of the malignant blasts.

Hyperpolarisation‐activated cyclic nucleotide‐gated channels regulate the spontaneous firing rate of olfactory receptor neurons and affect glomerular formation in mice

Olfactory receptor neurons (ORNs), which undergo lifelong neurogenesis, have been studied extensively to understand how neurons form precise topographical networks. Neural projections from ORNs are principally guided by the genetic code, which directs projections from ORNs that express a specific odorant receptor to the corresponding glomerulus in the olfactory bulb. In addition, ORNs utilise spontaneous firing activity to establish and maintain the neural map. However, neither the process of generating this spontaneous activity nor its role as a guidance cue in the olfactory bulb is clearly understood. Utilising extracellular unit-recordings in mouse olfactory epithelium slices, we demonstrated that the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels in the somas of ORNs depolarise their membranes and boost their spontaneous firing rates by sensing basal cAMP levels; the odorant-sensitive cyclic nucleotide-gated (CNG) channels in cilia do not. The basal cAMP levels were maintained via the standing activation of β-adrenergic receptors. Using a Tet-off system to over-express HCN4 channels resulted in the enhancement of spontaneous ORN activity and dramatically reduced both the size and number of glomeruli in the olfactory bulb. This phenotype was rescued by the administration of doxycycline. These findings suggest that cAMP plays different roles in cilia and soma and that basal cAMP levels in the soma are directly converted via HCN channels into a spontaneous firing frequency that acts as an intrinsic guidance cue for the formation of olfactory networks.

Acute effects of hexabromocyclododecane on Leydig cell cyclic nucleotide signaling and steroidogenesis in vitro

Hexabromocyclododecane (HBCDD), an additive brominated flame retardant routinely added to various consumer products, was reported to have toxic effects upon biota, including endocrine disruption. In this study, the potential toxicity of HBCDD was tested in peripubertal rat Leydig cells in vitro during 6h exposure. HBCDD inhibited human chorionic gonadotropin- and forskolin-supported cAMP accumulation and steroidogenesis. It also inhibited basal cAMP production, but elevated basal steroidogenesis. The expression of several cAMP-dependent genes, including steroidogenic acute regulatory protein, cholesterol side chain cleavage enzyme, and 3β-hydroxysteroid dehydrogenase, was also inhibited by HBCDD treatment. Nevertheless, this was not accompanied by a decrease in steroidogenic acute regulatory protein expression, as documented by western blot analysis, and activity of steroidogenic enzymes, as documented by unaffected steroidogenesis in the presence of permeable 22(R)-hydroxycholesterol. However, HBCDD caused significant decrease in mitochondrial membrane potential in untreated and human chorionic gonadotropin-treated cells. This indicates that HBCDD acute toxicity in Leydig cells reflects changes in mitochondrial membrane potential-dependent cAMP production and basal and cAMP-regulated cholesterol transport. This in turn facilitates basal but inhibits cAMP-dependent steroidogenesis. Acute effects of HBCDD treatment on transcription are also indicative of its sustained effects on Leydig cell function.

Intragenic suppression of a constitutively active allele of Gsα associated with McCune–Albright syndrome

McCune-Albright syndrome (MAS) is a human genetic disorder caused by a mutation that constitutively activates the G(s)α subunit by abolishing GTP hydrolysis. MAS patients suffer from a range of endocrinopathies as well as polyostotic fibrous dysplasia of bone. We previously identified an intragenic suppressor of the MAS mutation in a yeast system, which substituted two residues in the GTP-binding site of Gpa1: L318P and D319V to suppress the constitutive activity of an R297H mutation, corresponding to the human F222P, D223V, and R201H mutations respectively. To extend these studies, the human GNAS gene was subjected to site-directed mutagenesis. Constructs expressing the MAS mutation (R201H), the MAS mutation plus the mutations homologous to the yeast suppressors (R201H, F222P/D223V), or the yeast suppressor mutation alone (F222P/D223V) were transfected into HEK293 cells, and basal and receptor-stimulated cAMP levels were measured. Expression of R201H increased the basal cAMP levels and decreased the EC(50) for hormone-stimulated cAMP production. These effects were dependent on the amount of R201H protein expressed. R201H, F222P/D223V abolished the constitutive activity of the MAS mutation and caused responses to hormone that were not different from those measured in cells expressing WT G(s)α. Interestingly, F222P/D223V behaved similar to R201H in causing increases in basal cAMP production, thus demonstrating constitutive activity. Substitution of another acidic (E) or polar (N, T, and G) amino acid at position 223 caused no suppression of R201H activity, while substitution of a second nonpolar amino acid (A) at this position partially suppressed, and the larger polar I residue completely suppressed the effects of R201H.

Regulation of CAMP Compartmentation by Membrane Microdomains

The role of different membrane domains in the compartmentation of cAMP signaling was investigated using the FRET-based biosensor Epac2-camps. The MyrPalm sequence from Lyn kinase and the CAAX sequence from Rho GTPase were used to target this probe to lipid raft (Epac2-MyrPalm) or non-lipid raft (Epac2-CAAX) domains, respectively. Confocal imaging established that both probes were targeted to the plasma membrane in HEK293 cells. FRAP analysis demonstrated that depletion of membrane cholesterol altered both the recovery half-time and the mobile fraction of Epac2-MyrPalm but not Epac2-CAAX, confirming that each probe was targeted to the correct microdomain. FRET responses of these probes were then used to monitor relative changes in cAMP activity associated with lipid raft and non-raft domains. The results demonstrated that basal cAMP activity is significantly higher in non-raft domains. This was supported by the fact that the maximal increase in cAMP over baseline following agonist stimulation was significantly smaller for Epac2-CAAX than it was for Epac2-MyrPalm, consistent with the idea that the probe was partially activated by a higher basal level of cAMP associated with non-lipid raft domains. In addition, inhibition of adenylyl cyclase activity with MDL 12330A reduced basal cAMP activity detected by Epac2-CAAX but not Epac2-MyrPalm. Responses detected by Epac2-CAAX were also more sensitive to direct activation of adenylyl cyclase by forskolin, but less sensitive to inhibition of type 4 phosphodiesterase activity by rolipram. These results indicate that there are diffusionally-restricted pools of cAMP associated with different membrane microdomains under basal conditions. The higher basal cAMP activity associated with non-lipid raft domains can be explained by differences in basal adenylyl cyclase and phosphodiesterase activity.

Histamine Modulates Isoproterenol Efficacy at the β2 Adrenoceptor: Inferences Regarding Allosteric Modulation by Imidazole-Containing Compounds

In the last few years, evidence has been mounting of the possible allosteric modulation of second messenger formation on β2-adrenoceptors (β2ARs) induced by non-arylethylamine compounds. We herein addressed this issue by carrying out functional and molecular modeling studies to explore the possibility that imidazole-containing compounds (ICCs) affect the formation and accumulation of cAMP by mediating β2AR activation. Results show that histamine (an ICC) had no effect on basal cAMP accumulation in COS-7 cells transfected with the human β2AR (10.4 ± 1.1 pmol/mg protein), but significantly augmented the receptor response to the agonist isoproterenol (137 ± 7% of controls, EC50 10.5 μM, pEC50 5.87 ± 0.06). Moreover, histamine (10 μM) did not affect the isoproterenol inhibition of 3H]-dihydroalprenolol binding to hβ2ARs in membranes of transfected COS-7 cells. Q-site Finder program and molecular docking studies identified possible sites of interaction for ICCs on the β2AR, but the estimated affinities were lower than those reported for well-known β2AR ligands on the orthosteric site. On the basis of the present experimental and theoretical data, we postulate that direct ICC-hβ2AR interactions can allosterically modulate agonist-induced receptor activation. Implications of these findings for drug design are discussed.

Relaxin and Sertoli cell proliferation.

Immature Sertoli cells proliferate and several factors affect their number, including the follicle stimulating hormone (FSH), testosterone, estradiol and several paracrine growth factors. Using a primary culture of Sertoli cells isolated from 15-day old Wistar rats we have shown that relaxin stimulates Sertoli cell proliferation through the activation of MEK/ERK1/2 and PI3K/AKT pathways. In contrast, FSH inhibited both ERK1/2 and AKT phosphorylation. Furthermore, FSH strongly increased cAMP production, whereas relaxin inhibited basal cAMP production. Our results indicate that in rat Sertoli cells from 15-day old rats relaxin and FSH affect the same signaling pathways in opposite directions. Interplay between both hormones may be important to control the proliferation and differentiation of Sertoli cells.